The obstetrical significance of maternal urogenital carriage of group B streptococci (GBS) was investigated in a prospective study comprising 799 parturients and their infants. 128 mothers were GBS carriers. Fetal heart rate recording showing abnormal baseline frequency and concomitant late decelerations during delivery were found in 6 infants born of GBS carriers (5%) and in 7 (1%) of non-carriers (P < 0.01). The pH of fetal blood was determined in 14 (11%) GBS carriers and in 53 (8%) non-carriers. 6/14 (43%) infants of GBS carriers had pH values below 7.20, in contrast to 4/53 (8%) of the non-carrier group (P < 0.01). Mothers of infants showing signs of intrauterine asphyxia had the same amount of antibodies to GBS as mothers of infants without signs of asphyxia. Among the GBS carriers, 11/128 (9%) were delivered later than the 42nd week of pregnancy. A smaller proportion of non-carriers, 21/671 (3%), were delivered postterm (P < 0.01).